1. Field of the Invention
This invention relates to inverters, and more particularly to a transistor inverter base drive circuit.
2. Background Art
FIG. 1 shows the arrangement of a conventional transistor inverter base drive circuit which is used to convert a DC voltage to an AC voltage. In FIG. 2, a first power source 1 supplies a forward base current. There are a photo-coupler 2, base drive transistors 3 through 7, an inverter main circuit transistor 8 (which is in the form of a Darlington transistor), a base current limiting resistor 9, and diodes 10 through 18. A second power source 19 supplies a reverse base current. There are further a capacitor 20, an inductor 21 and resistors 22 through 32.
The operation of the transistor inverter base drive circuit thus organized will now be described.
A pulse signal is provided by a control circuit P to control the switching of the transistor 8 in order to switch the direction of current flow, thus converting a DC voltage to an AC voltage. This pulse signal is transmitted through the photocoupler 2 to drive the PNP transistor 3 and the NPN transistor 4 which are provided at the rear stage of the photocoupler 2. The NPN transistor 5 is connected as a rear stage to the transistor 4 so that the transistor 3 and 5 operate as a complementary pair. That is, when the transistor 3 is rendered condutive (on), the transistor 5 is rendered non-conductive (off), and vice versa.
The case where the transistor 3 is conductive will be described. The base current IB.sub.1 of the inverter main circuit transistor 8 flows from the power source 1 through the transistor 3, the resistor 9, the diode 10, the transistor 7 and the inductor 21 to the base of the transistor 8, and returns through the series of diodes 11-16 to the power source 1. In this operation, a forward voltage corresponding to the six diodes 11-16 is charged in the parallel capacitor 20.
When the application of the pulse signal from the control circuit P is suspended, the transistor 3 is rendered non-conductive (off) while the capacitor 20 is discharged through the transistor 8, the inductor 21, the transistor 7, the transistor 6, the resistor 28 and the transistor 5. At the same time, the transistor 6 is turned on, and a reverse bias current IB.sub.2 flows from the power source 19 to the transistor 8 and the transistor 7, thus rendering the transistor 7 non-conductive (off). In this operation, the diode 18 serves as a bypass after the transistor 7 is turned off.
The conventional transistor inverter base drive circuit is organized as described above. Therefore, it is necessary that the voltage required for rendering the transistor 6 conductive (on) is charged in the capacitor 20, and the forward voltage drop of the diodes 11 through 16 connected in parallel to the capacitor 20 is utilized for the charging of the capacitor 20. Therefore, it is necessary to use a relatively large number of diodes 11-16. Therefore, the circuit is intricate, and the forward current flowing in the diodes generates heat. That is, the circuit has a low thermal reliability.